Are NDAs unenforceable when they protect more than trade secrets? The standard answer is no. NDAs can prevent disclosure of contractually-defined "confidential" information that is shared in the course of a confidential relationship, even if it is not technically a trade secret. NDAs can, in other words, go beyond trade secrecy.
NDAs have also not traditionally been treated as contracts in restraint of trade, like noncompetes are. An NDA's purpose is, ostensibly, just to protect secrets. Similar to trade secret law, NDAs only prevent an employee from disclosing (and using outside authorization) specifically-defined information. They don't prohibit competition per se. NDAs are thus seen as comparatively "narrow restraints" which, all else being equal, should be preferred to noncompetes.
Or at least that is the common wisdom. Although there is some support for this viewpoint in treatises and judicial dicta, our new article, Beyond Trade Secrecy: Confidentiality Agreements That Act Like Noncompetes, shows that a growing contingent of courts across jurisdictions are finding NDAs in employment agreements to be unenforceable when they reach too far beyond trade secrecy. Even Google's NDA was recently found unenforceable by a California court, because it did not make sure employees could use or share skills they learned at Google with prospective employers. (That said, the Google opinion is quite extreme, even compared to others we reviewed. See pp. 8-11 of the opinion, Doe v. Google, Inc., Case No. CGC-16-556034 (Cal. Super. Ct., Cty. of San Francisco, Jan. 13, 2022)).